Here's the answer, from Garner's "Modern American Usage", 3d ed.: The indefinite article a is used before words beginning with a consonant sound, including /y/ & /w/ sounds. The other form, an, is used before words beginning with a vowel sound. Since the sound rather than the letter controls, it is not unusual to find a before a vowel or an before a consonant. And here, the following word, "uniform", begins with a "y" sound; therefore, a is appropriate.
thanks. I understand. U is pronounced as YOU, hence the "A" . I learned something. Learning Russian with an English basis is helping me refresh my English, a language I am not practicing since 1992 because I live in South America and I use Spanish and French ( my mother-tongue) almost excliusively.,
FYI, "jersey" was accepted when used in the phrase: Футбольная форма